Wireless communication systems are well-known in the art. In such systems, a plurality of subscriber units (such as in-hand portable and/or vehicle-mounted mobile radios) are provided communication services via wireless communication with a fixed infrastructure. The fixed infrastructure typically comprises a plurality of sites in communication with each other. Each site has a corresponding coverage area. Any subscriber units currently operating within the coverage area of a given site are serviced by that site. As subscriber units roam throughout a system, they can leave the coverage area of one site and enter the coverage area of another site. In order to provide seamless service throughout multiple coverage areas, subscriber units must determine (or be instructed) when to change affiliations between sites.
In order to determine when to de-affiliate with a current site and affiliate with another site, it is known in the art for subscriber units to maintain site rankings for various sites. Site rankings are used to compare the relative "quality" of service provided by various sites. Thus, each subscriber unit continuously maintains and assesses site rankings in order to select the best possible site with which to affiliate. A variety of factors are measured and/or tracked in order to determine each site ranking maintained by a subscriber unit. Examples of such factors include, but are not limited to: the received signal strength at the subscriber unit for outbound transmissions from a given site; whether the subscriber unit has ever been denied access to the given site; whether a given site is geographically adjacent to a subscriber unit's current site; and the bit error rate at the subscriber unit for outbound transmissions from the given site. As these examples demonstrate, current techniques for assessing site rankings are generally based on outbound transmissions (i.e., from a site to a subscriber unit), and not inbound transmissions (i.e., from a subscriber unit to a site). This technique is acceptable in those instances where outbound transmission quality is equivalent to inbound transmission quality, as would be the case in an ideal, well-designed system. In the present context, equivalence in quality is measured by equivalence in communication range; equivalence occurs when the outbound range of given site is equal to the inbound range. In such a system, a subscriber unit at the edge of a given site's coverage area will be able to transmit and receive communications equally well.
However, in practice it is often that case that inbound transmission quality is not equivalent to outbound transmission quality. This can be a result of several causes. For example, the overall system could be poorly designed. Alternatively, the system could have been designed for use only by mobile radios having relatively high transmit powers, but where some lower power portable radios were added later. In yet another instance, variances in terrain surrounding a site's transmitter and receivers (which may not necessarily be co-located) can causes differences in coverage. Regardless of the cause, differences in outbound and inbound ranges and current site ranking techniques that rely solely on outbound measurements can cause a subscriber unit to affiliate with a site that has a good site ranking due to its outbound coverage, but that provides inadequate inbound coverage. Therefore, it would be an advance over the current state of the art to incorporate inbound quality measurements into site ranking determinations.